for an external monitor on a Mac as, for the most part, monitors are platform agnostic and only require the correct video signal (a standard that all computer manufactures follow) to be input to it.
Take a look at the keyboard and see if you can find the output key, it is usually a key that requires the fn key to be pressed at the same time. I cannot guarantee that there is one, I have nothing to check with.
Have you tried all the options in the System Preferences/Displays?
Has the monitor got a DVI input as well as a VGA input?
I have an HP LP2465 Monitor. If I log on to the HP web page it appears that their are only Windows drivers and no reference to Mac compatability. Perhaps, this monitor may not be compatable with the Macbook? When I'm in mirror mode, the Macbook recognizes the HP LP2465 Monitor, but doesn't activate the monitor through the Mini-DVI to VGA Adapter. The monitor just goes to sleep. Does anyone know of anyway to get Macbooks to work with HP Windows monitors? Are there drivers available for the Apple?